Lithium-ion secondary batteries have a combination of high energy and high power density, making it the technology of choice for portable electronic devices, power tools, electric vehicles (“EVs”), energy storage systems (“ESS”), cell phones, tablets, space applications, military applications, and railways, electric vehicles (EVs), include hybrid electric vehicles (“HEVs”), plug-in hybrid electric vehicles (“PHEVs”), and pure battery electric vehicles (“BEVs”). If EVs replace the majority of fossil fuel (e.g., gasoline, diesel fuel, etc.) powered transportation, lithium-ion secondary batteries will significantly reduce greenhouse gas emissions. The high energy efficiency of lithium-ion secondary batteries may also allow their use in various electric grid applications, including improving the quality of energy harvested from wind, solar, geo-thermal and other renewable sources, thus contributing to their more widespread use in building an energy-sustainable economy.
Therefore, lithium-ion secondary batteries are of intense interest for commercial ventures as well as in basic research in government and academic laboratories. Although research and development in this field has abounded in recent years and lithium-ion secondary batteries are currently in use, there remains a need for improvements with respect to higher capacity, higher current generation, and batteries that can undergo more charge/discharge cycles thereby extending their useful life. Additionally, improvements in the weight of the batteries are needed to improve applications in several environments, such as vehicle, portable electronics and space applications.
Lithium-ion secondary batteries typically include a negative electrode current collector of a metal foil on which is deposited a negative electrode active material. Copper foils are often used as the negative electrode current collector because copper is a good conductor of electrical current. As demands for lower weight batteries become ever more urgent, the current collector needs to be thinner to reduce the size and weight of lithium-ion secondary battery. These thinner current collectors are prone to wrinkling, tearing, cracking and other forms of damage. Additionally, to increase the capacity of the lithium-ion secondary battery, materials such as silicon (Si), germanium (Ge), and tin (Sn) are mixed with or fill the higher capacity active material in a battery, exacerbating the expansion and contraction of the active material and stresses on the copper foil it is in contact with. Furthermore, in some recent advancements, in order to increase the capacity of the batteries, the copper foils, worked as electrodes, are folded and wound. If the copper foil cannot withstand the expansion and contraction of the active material during battery use, and folding and winding during production of the battery, the cycle characteristics of the battery are adversely affected.
There therefore remains a need for improved copper foils for use in lithium-ion secondary batteries. This includes a need for thinner copper foils having improved workability and durability and that, when combined with the negative electrode active materials to provide lithium-ion secondary batteries, do not fail under high cycles of charge and discharge due to separation between the copper foil and the electrode active materials, or fail due to the copper foil fracturing. All the while these needed thinner copper foils must fulfill the goals of reducing the weight and increasing the capacity of lithium-ion secondary batteries.